1. Field of the Invention
This invention relates to an optical axis correcting apparatus and a method of correcting an optical axis, and more particularly, is suitably applied to an optical axis correcting apparatus of an optical space transmission system that spatially transmits light beams.
2. Description of the Related Art
A transmission system transmits data from the transmitting side to the receiving side through a cable circuit or by radio. A construction to transmit data from transmitting side to receiving side through a cable circuit has ways in which the cable circuit is provided virtually and underground. However, both methods require troublesome procedure and large-scale construction. On the other hand, to transmit data from the transmitting side to the receiving side by radio, a prescribed frequency band must be allocated out of a limited electric wave source, and realizing it is difficult because practically there is limitation in the number of circuits.
Then, in recent years, an optical space transmission system that transmits various data in optical space with an optical circuit using light beams has been developed. However, an optical space transmission system having sufficient performance to transmit data to a long distance without error has not been developed yet.
For example, as shown in FIG. 1, the optical system 100 of an optical space transmission system capable of the bi-directional communication converts a laser beam from a semiconductor laser 101 that has been modulated in intensity based on a transmission signal into a parallel beam with a lens 102, and makes the parallel beam be incident into a beam splitter 103. The beam splitter 103 reflects the parallel beam and makes it be incident on a concave lens 104 to magnify the parallel beam. Then, the magnified beam is converted into a parallel beam again through a convex lens 105 and is emitted as an emitted light L.sub.out.
Furthermore, the optical system 100 brings an incident light L.sub.in which is transmitted from the communicating party of an optical space transmission system into convergence on the concave lens 104 with the convex lens 105. The converged light is converted into a parallel beam by the concave lens 104, and then the parallel beam is incident into a beam splitter 106 through the beam splitter 103. The beam splitter 106 reflects the parallel beam and brings it into convergence on the light receiving surface of a position detecting sensor 108 through a lens 107. At the same time, the beam splitter 106 makes the parallel beam pass through the sensor 108 and brings it into convergence on the light receiving surface of a light receiving device 110 through a lens 109.
In such an optical system 100, the optical axes must be identical with each other between that system and the optical system of the optical space transmission system of the communicating party. However, deviation unfortunately occurs between their optical axes because the optical system receives influences such as external causes, such as fog, rain, etc., an oscillation occurred inside the system, the change of a temperature of a setting place, or the like. In this case, in the optical space transmission system, even a slight deviation of optical axis causes an error in optical space transmission to a long distance, and it obstructs the communication.
To correct such the deviation of the optical axis, various optical axis correcting apparatuses have been provided. For example, as shown in FIG. 2, in an optical axis correcting apparatus 120, the aforementioned optical system 100 is integrally provided in a body tube 121. The above body tube 121 is supported by an intermediate ring 122 with two bearings for X-axis 123 so as to freely rotate on the X-axis in a rotary-direction.
A motor for X-axis 124 is fixed to the intermediate ring 122. The above motor for X-axis 124 transmits its rotary driving power via a driving gear 125 to a driven gear 126 that is integrated with the bearing for X-axis 123. This makes the body tube 121 rotate on X-axis in the rotary-direction.
Furthermore, the intermediate ring 122 is supported by a pedestal 127 with a bearing for Y-axis 128 so as to freely rotate on Y-axis in the rotary-direction. A motor for Y-axis 129 is fixed to the pedestal 127. The motor for Y-axis 129 transmits its rotary driving power via a driving gear 130 to a driven gear 131 which is integrated with the bearing for Y-axis 128. This makes the intermediate ring 122 and the body tube 121 integrally rotate on Y-axis in the rotary-direction.
The motor for X-axis 124 and the motor for Y-axis 129 make the body tube 121 rotate by a prescribed amount based on the detected result of the position detecting sensor 108 (FIG. 1) with a control part (not shown in figure) such that the optical axis of the emitted beam L.sub.out in transmission and the optical axis of the incident beam L.sub.in in reception are identical with each other.
On the other hand, as shown in FIG. 3 in which the same reference numerals are applied to corresponding parts of FIG. 1, the optical axis correcting apparatus 140 is composed of a mirror for X-axis 141 provided on the optical path of the optical system 100, a motor for X-axis 142 which makes the mirror for X-axis 141 rotate on X-axis in the rotary-direction, a mirror for Y-axis 143 provided at a position opposite to the mirror for X-axis 141, and a motor for Y-axis 144 which makes the mirror for Y-axis 143 rotate on Y-axis in the rotary-direction.
In this case, the optical axis correcting apparatus 140 makes each of the motor for X-axis 142 and the motor for Y-axis 144 rotate by the prescribed amount based on the detected result of the position detecting sensor 108 with the control part (not shown). This adjusts the rotary angles of the mirror for X-axis 141 and the mirror for Y-axis 143 such that the optical axis of the emitted beam L.sub.out in transmission and the optical axis of the incident beam L.sub.in in reception are identical with each other.
In the-mentioned optical axis correcting apparatus 120 (FIG. 2), since the optical axes are corrected by moving the whole body tube 121, there is a problem that a response to a command to correct an optical axis deteriorates by the inertia mass of the whole body tube 121.
Furthermore, the optical axis correcting apparatus 120 has problems that accurate bearings and motors for generating large driving power are needed and that the optical axis cannot be accurately corrected because of various influence of its transmission mechanism owing to the motor for transmit rotary power and backlash of gears.
Also the optical axis correcting apparatus 140 (FIG. 3) requires a mirror and a motor for each of X-axis direction and Y-axis direction. This causes problems that its configuration is complicated and enlarged and that the optical axis cannot be accurately corrected owing to backlash in its transmission mechanism.
Furthermore, in the optical axis correcting apparatus 120 and the optical axis correcting apparatus 140, in the case where the rotary angles of the body tube 121, the mirror for X-axis 141 and the mirror for Y-axis 143 are controlled only by their positional information (i.e., angles), the body tube 121, the mirror for X-axis 141 and the mirror for Y-axis 143 unfortunately move from the stop positions when given some large oscillation from outside. Thus, stable control cannot be performed.
The optical axis correcting apparatus 120 and optical axis correcting apparatus 140 are provided with speed sensors which respectively detect an angular velocity component having a high frequency of the time when oscillation leads to movements of the body tube 121, the mirror for X-axis 141 and the mirror for Y-axis 143. The angular velocity component which represents the actual movement detected by the speed sensor is fed back to restrain the movement owing to the oscillation component. Thus, the body tube 121, the mirror for X-axis 141 and the mirror for Y-axis 143 can be controlled stably. However, in the case where the speed sensors are provided individually in the optical axis correcting apparatuses 120 and the optical axis correcting apparatus 140, there is a problem that their configurations are complicated and the whole apparatuses are enlarged.